Sligo Bay Cuan Shligigh

Overview

Sligo Bay is situated in Co. Sligo in the northwest of Ireland. Sligo Bay can be defined as an area east of a line made by the headlands at Aughris Head and Raghly Point. The distance between these two features is around 9 kilometres. Inside of these headlands are three smaller bays. These are Ballysadare Bay, Sligo Harbour and Drumcliff Bay. Two of these smaller bays, Ballysadare and Drumcliff, are partially separated from the outer bay by spit features with large areas of tidal flats that are exposed at low tide.

Coney Island, Manguin’s Island and Oyster Island lie between Sligo Harbour and the outer bay. These intertidal areas are ideal habitats for shellfish whose abundance have supported humans inhabitation in the area since the Stone Age, evidenced by the large midden mounds which are found at various sites along the coast. These shells give Sligo its Irish name of Sligeach meaning ‘the place of shells’. INFOMAR surveys have also mapped the approaches to Sligo Bay.

INFOMAR Survey History

In 2002 the Celtic Voyager surveyed a large portion of the deeper waters of outer Donegal Bay as part of a larger survey area extending west to Erris Head, Co. Mayo. In early 2008, the Celtic Voyager was again used to survey the areas close to the coast all around Donegal Bay.

Later in the summer of 2008, BLOM Aerofilms flew a lidar survey over Sligo bay.

A smaller survey was carried out by the RV Keary in 2014 around Inishmurray Island.

Coverage from survey legs of Sligo Bay.(Click image for more detailed map).

Shaded Relief

The 3D appearance is achieved using software called Fledermaus. By using some vertical exaggeration, artificial sun-shading (usually as if there is a light source in the nw 315°) and colouring the depths using various colour maps, it is possible to highlight the subtle relief of the seabed. This helps us to quickly understand the variation in depths.

MBES shaded relief overview image of the area mapped in Sligo Bay and approaches. (Click image for more detailed map)

Backscatter

Multibeam Systems also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head.

Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.

MBES backscatter image of the same area seen in shaded relief (above) off Raghly Point, Co. Sligo where varying the various shaded of grey represent the intensity of sound reflected from the seabed recorded by the multibeam sensors depending on hardness and other physical characteristics. The dark areas are rock outcrops and coarse sediments with fine sediments represented by light grey. (Click image for more detailed map).

LiDAR

In the summer of 2008, BLOM Aerofilms flew a lidar survey over Sligo bay.

LiDAR coverage in Sligo Bay from 2008 lidar survey. Interesting aspects include the sinuous tidal channels running through Ballysadare Bay in the south and also the navigation channel out of Sligo Harbour.

To view and navigate around the Lidar Sligo Bay dataset in Google Earth, click here

Bathymetry and shaded relief charts of the Lidar coverage can be downloaded here.

Ground Truthing

A range of seabed sampling has been undertaken in Sligo Bay, both historically and under the INFOMAR project. The locations of the grab samples have been mainly determined by the seabed classification made from the multibeam data.

Complete map of sampling locations of the northwest coast of Ireland including Sligo Bay and Approaches. (Click image for more detailed map)

Seabed samples were taken in 2002 by the RV Celtic Voyager in the the deeper waters of outer Donegal Bay (blue circles).

From the 6th to 8th of July 2009, a ground truthing or seabed sampling survey was undertaken in the region of Donegal and Sligo Bay. 80 samples were collected using a day grab (pink circles).

Seabed Classification

The classification of the multibeam dataset from Sligo and Donegal Bays resulted in the creation of a 6 class classification divided into two types of rock, reflecting the different textures observed from rock outcrops in the bay. Three more classes divided the sediments into Fine Sand to Mud, sand and a sand veneer over rock. The final class displays areas of soft ground but showing high backscatter, possibly driven by biogenic content.

Seabed Habitats

The seabed substrate information has been derived from a combination of the analysis of geophysical data (multibeam echosounder bathymetry and backscatter) and groundtruthing data acquired as part of the INSS and INFOMAR seabed mapping programmes.

Geology

Sligo Bay Geology

The rocks of Sligo Bay’s coastline are primarily composed of limestone and shale, with lesser exposures of mudstone and sandstone. Limestone is a carbonate rock that would have been formed many millions of years ago in a warm, shallow sea, whereas shale would originally have been formed from layers of mud in a deeper marine setting.

The Bedrock datasets used in the Geology Map above are the Onshore Geology map Bedrock Geology 1:1 Million scale map produced by the GSI for the OneGeology project. It is available as a WMS. The Seabed geology map was compiled for WP4 of the EMODnet Geology project (lithology). It is available as a WMS and the Irish data can be downloaded as instructed below.